The ASIO specification requires the host (SONAR in this case) to use one ASIO driver at a time. To change drivers both input and output must be deselected before SONAR will enable all drivers. Once an audio input or output is selected everything except that driver is disabled. This is how SONAR has always worked when using ASIO driver mode. It must work this way to satisfy the ASIO spec.
 
To use more than one driver at a time either select a different driver mode in SONAR preferences (try one of the WASAPI settings in Win10) or use ASIO4ALL (this actually uses the WDM drivers so may want to try WDM in SONAR first). This must be how the previous installation was set up.

-francesco-
The problem is that what I actually needed (and this may be the central point) was to use my focusrite to record my keyboard, AND THEN HEAR THE SOUND OUT OF MY SPEAKERS. So, in this moment, I don' t want sound to come out from my Focusrite (using amps or speakers connected to it) but from the speakers of my pc, the same ones I used to listen to any normal audio file, and wich I used to listen to everything into my daw until now...

 
Connecting the SPDIF Out from the Focusrite to the Reltek's SPDIF In would solve this.

 
Sometimes things gets lost in Translation. :)
 

FINALLY SOLVED IT!!!!!! I did it by switching in WASAPI drivers and now everything works fine. It didn' t think about that as the solution because I used ASIO drivers in my previous device, but...is there anyt difference between ASIO and ASIO4all? Because that' s ASIO4ALL I was using, and I didn't thought there actually was a difference.
I began to believe that the solution would have not been simple as I thought at all, but now everything is fine.
 
Thank you all :)

ASIO4ALL is a wrapper for WDM drivers. It was originally designed to connect software requiring an ASIO driver to hardware that only had a WDM driver. One of the features of ASIO4ALL is the ability to use multiple WDM drivers and present them as a single ASIO driver to the host. In most cases it is preferable to use factory supplied ASIO drivers but this usually means access to one device at a time.

Just be warned that way you have set up your audio and driver possibly will cause your overdubbed tracks to be out of sync. Real  ASIO drivers have the ability to properly calculate the timing offset. All other driver modes do not. So when you lay down a new track it can be late in timing to the original song.